Heating means and method



J. M. BEASLEY ETAL HEATING MEANS AND METHOD Original Filed Aug. 11 1961N mL' Feb. 22, 1966 E-; .n .u

L /mw IN VENTORS A G E N T Ov Om JACK M. BEASLEY HERBERT GREENEWALD,JR.

Reissued Feb. 22, 1966 25,958 HEATING MEANS AND METHOD Jack M. Beasley,Grand Prairie, Tex., and Herbert Greenewald, Jr., Columbus, hio,assignors, by mesne assignments, to Ling-Temco-Vought, Inc., Dallas,Tex., a corporation of Delaware Original No. 3,106,594, dated Oct. 8,1963, Ser. No. 130,830, Aug. 1l, 1961. Application for reissue Mar. 15,1965, Ser. No. 455,025

11 Claims. (Cl. 13-1) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

This invention relates to methods for electric heating, and moreparticularly to a method for heating an enclosure with a plasma.

High-temperature electric furnaces have previously fallen into fourprincipal groups when classified according to the method of heatingemployed. These four groups have included arc furnaces; furnacesemploying a solid resistance element; furnaces employing a liquidresistance element; and electron-beam furnaces. All these devices havehad certain disadvantages and limitations, and each feature making anyone ot them attractive for a given, particular utilization is generallyoffset by attendant and previously unavoidable disadvantages.

Thus, while the current flow to a furnace employing a solid or liquidresistance element is easily stopped and restarted as required toprovide the alternate periods of cooling and heating needed formaintaining the furnace interior within a desired temperature range. thesolid resistor furnace is limited in operation to the temperature atwhich the resistor melts or begins to experience serious chemical atackby the atmosphere of the furnace chamber, while a liquid resistorfurnace can be heated no further than the temperature at which theliquid resistor vaporizes. While fairly easy to re-start after a periodof operation, an electron beam furnace nonetheless presents seriousdifficulties in temperature control and is operable only under arelatively very high vacuum` Current flow to an arc furnace is easilystopped by opening a switch, but the narrow, ionized zone forming theconducting medium between the electrodes disappears immediately uponcessation of the electrical flow. and the current cannot be restartedsimply by closing the switch. Instead, the electrodes must be moved intocontact with each other and then separated slightly to draw the are; orelectrical equipment must be supplied which will yield a specialstarting voltage high enough to provide an initial spark` across theelectrode gap. 'Temperature control thus tends to be difficult andunwieldy in an arc furnace.

All the previously employed furnaces have been beset with thedisadvantage of large temperature gradients within the furnace chamberwhich seriously limit furnace cfciency. This problem is especiallycritical in electron beam furnaces and arc furnaces and is alleviated ina solid resistor furnace only by making the resistor area quite large inrelation to the furnace interior and to the electrical power input.Temperature gradients between a liquid resistor and the material to beheated in the furnace are undesirably large except where the material lobe heated can be immersed in the liquid or is melted to itself form theliquid resistor. In the case of an arc furnace, all the heat isgenerated in the small region including the electrode tips and the arcbetween them, with most of the heat originating in the electrode tipsrather than in the arc. As in the case of a solid or a liquid resistorfurnace, heat distribution in an arc furnace must be by radiation,convection, and conduction, and the etilciency of heat distribution fromthe small zone of heat origination in an arc furnace therefore isundesirably low. This undesirability is further aggravated where directcurrent is employed in an arc furnace, for such operation results instill further localization of the zone of origin of the heat in that theanode tip produces twice as much heat as the cathode.

Further dinculties arising in the operation of arc furnaces are relatedto the unavoidable occurrence of electrode deterioration. When underD.C. operation of graphite electrodes, vaporized carbon passes from thecathode to the anode and is deposited on the tip of the latter. Thiscarbon button interferes with arc propagation and all too frequentlyfalls away from the anode into the furnace charge, into which it entersas a contaminant. In addition, the current density at the electrode tipsbecomes excessively high under operation with either direct oralternating current, and the resulting high rate of consumption of theelectrode material fills the furnace with vapors which contaminate thematerial heated in the furnace.

It will be evident that it is `most desirable to provide a method ofoperating a furnace yielding advantages of previous furnace operatingmethods while obviating their disadvantages.

It is, accordingly, a major object of the prescrit invention to providegreatly improved uniformity of temperature within an electric furnace.

Another object is to provide improved ease and ciliciency in temperaturecontrol of an electric furnace without resort to a solid or liquidresistor element or the necessary utilization of a high vacuum.

A further object is to provide for the attainment of higher temperaturesthan are possible in :i solid or liquid resistor furnace while obtainingimproved temperature distribution in the furnace chamber and ctlcicnttemperature control without the need for moving the furnace electrodesor employment of special starting voltages.

Yet another object is to provide furnace operation wherein an inert,gaseous plasma hlls the furnace chamber and serves as the electricalresistance element.

A still further object is to reduce greatly the deterioration of theelectrodes in an electric furnace and the contamination of the furnacecontents by vaporized or deposited portions of the electrodes.

Still another object is to provide purging and washing of a melt in afurnace chamber by an inert gas which serves as a plasma resistanceelement filling the furnace chamber.

Other objects and advantages will be apparent from the specification andclaims and from the accompanying drawing illustrative ofthe invention.

In the drawing:

FIGURE l is a front elevation, in central longitudinal section, of afurnace suitable for practice of the present invention` the electrodesbeing shown in position for preheating the furnace interior;

FIGURE 2 is a view similar to FIGURE l but only partially in section toshow the gas-tight door of the mold compartment and further showing amodifuation for effecting temperature control of the furnace, theelectrodes being positioned for plasma resistance operation of thefurnace;

FIGURE 3 is a View similar to FIGURE 1 and showing a Vsecondmodification for effecting temperature control, a mold being shown inplace to receive the molten metal;

FIGURE 4 is an exploded View of the electrode holder and associatedparts;

FIGURE 5 is an oscilloscope trace of the voltage between electrodesduring A.C. arc operation; and

FlGURE 6 is an oscilloscope trace of the voltage bctwecn electrodesduring plasma resistance operation.

Briefly described, the invention comprehends the method of heating anenclosure comprising the provision and maintenance in the enclosure ofan atmosphere of substantial (for example, at least argon content. Thisargon-enriched atmosphere is heated until at least some thermalionization occurs generally throughout the chamber, whereupon a pair ofelectrodes are provided in the chamber with a spacing greater than themaximum gap over which, at a given operating potential and in air (or ina cold, argon-enriched atmosphere) an arc could be propagated. When theoperating potential is, for example, 40 volts, a convenient andeffective spacing of the electrodes is of the order of 6 inches,although a somewhat smaller spacing is not harmful and a larger spacing,where the furnace dimensions permit, is acceptable. The operatingpotential is applied across the electrode gap to obtain an electricalflow through the ionized material distributed throughout the enclosure,thus utilizing the enriched atmosphere as a resistance element.Temperature control is obtained as more fully described in laterparagraphs. The invention further comprises means for carrying out theabove method.

With reference to FIGURE l, the electric furnace comprises a crucible 10preferably made of a dielectric material or provided with a dielectriclining. The crucible is of porous construction in order to permit thepassage of a gas under pressure from its exterior surface, in particularfrom its bottom, to its interior cavity or chamber 11.

The Crucible 10 is contained in a housing 12 with walls and partitionsof metal or other heat-resistant material which enclose all the Crucibleexterior surface in an airtight manner. Spaced slightly above thehousing lower wall 13 is a transverse partition 14 upon which the bottomsurface of the crueible 10 rests. An opening 15 somewhat smaller thanthe diameter of the lower surface of the Crucible 10 is formed in thepartition 14 and is overlapped around all its periphery by the poroussurface of the crueible 10, which thus has communication with a plenumchamber 16 enclosed within the housing 12 between the housing lower wall13 and partition 14. The housing upper side or wall 17 is spacedslightly above the crucible 10 to form therebetween a space which, likethe space between the sides of the Crucible 10 and the housing sidewalls 18, 19, is filled with an insulating material 20, preferably aceramic, which seals off the outer surface of the crucible at its topand sides. Since the lower surface of the Crucible 10 is in turn closedoff by the housing lower wall 13, the housing 12, including the ceramicinsulating material 20, sealingly isolates all the exterior surface ofthe crucible from the atmosphere. The plenum chamber 16 is filled with aporous or loose insulating material such as spherical, hollow grains offused alumina 21 followed by an outer layer of rock Wool 72. The plenumchamber 16 is connectible with a source of an inert gas, specificallyargon, through a tube 22 and thus, in cooperation with the porouscrucible 10, is a means for maintaining an increased concentration ofargon in the Crucible. An opening 66 through the housing upper side 17and adjoining insulating material 20 communicates with the cruciblecavity 11 and permits withdrawal of melted materials from the Crucible10.

The supporting frame 23 includes a pair of vertically extending, fixedmembers 24, 25 whose upper ends are spaced to either side of the housing12. A pair of fittings 26, 27 are rigidly mounted on the housing 12, oneat each side of the Crucible 10, and each fitting 26 or 27 pivotallyengages a respective supporting member 24 or 25.

A passage 28 extends axially through the fitting 26 and through thehousing wall 18, insulation 2l), and crucible 10 into the crucibleinterior to permit the mounting and variable extension into the cruciblechamber 11 of an electrode 30. A second electrode 3l is similarlyextensible into the chamber through a similar, second 4 passage 29 atthe other fitting 27. The electrodes 30, 31 preferably are of suchlength that, with their inner ends in contact with each other, theirouter ends extend exteriorly of the fittings 26, 27.

FIGURE, 4 shows a representative one of the fittings 26, 27, whichincludes a bearing ring 32, insulating gasket 33, electrode holder 34,and end cap 35. The bearing ring 32 is of tubular construction withspaced, circular end flanges 37, 38. The innermost end flange 37 isrigidly mounted, as shown in FIGURE 1, on the housing, while the otherfiange 38 is drilled for attachment of the circular electrode holder 34.The latter has a central, tubular portion extending away from thebearing ring 32 and is provided with a lug 39 or equivalent forattachment of the electrical lead 40 through which electrical power issupplied to the associated electrode 30.

The electrode 30 has a snug, sliding fit in the electrode holder 34. Thepassage 28, where it extends through the bearing ring 32, is of largerdiameter than the electrode 30 and the same is true of the portion ofthe passage 28 extending, as may be seen in FIG URE. l, from the bearingring 32 into the crucible chamber 11. The electrode, therefore, isspaced from the wall of the passage 28 except at the holder 34, by closesliding contact with which it is afforded support and electricalconnection with the lead 40. The spacing of the electrode 30 from thewall of the passage 28 is sufficient to prevent arcing to the bearingring 32 at operating voltages.

Means electrically isolating the electrode holder 34 of the one fitting26 or 27 from that of the other include the insulating gasket 33 placedbetween the bearing ring 32 and electrode holder 34 and insulatingbushings 42 between the bearing ring flange 32 and the electrode holderstuds 41 which extend through the bearing ring flange 38. insulatingwashers 43 are placed between the bearing ring iiange 33 and nuts 44which are run down on the studs 41 to clamp the electrode holder 34 inairtight manner on the bearing ring 32.

A plurality of cap-mounting studs 45 extend outwardly from the face ofthe electrode holder 34 and engage corresponding openings in a flange ofthe end cap 35. When wing nuts 46 are tightened down on the studs 45,the end cap 35 is pulled into close, airtight engagement with theelectrode holder 34 and encloses the protruding outer end (see FIGURE 1)of the electrode 30. The passages 28, 29, thus may be sealed off, asshown in FIG- URE 2, from the atmosphere to permit furnace operationunder a partial vacuum. The caps 35, 36 must be long enough to house theelectrode outer ends when the electrodes 30, 31 are fully separated, aswill be described, for plasma resistance operation.

Each supporting member 24 or 25 terminates at its upper end, as shown inFIGURE 4, in a lower trunnion half 47 which lies between the fianges 37,38 of and receives the tubular portion of the bearing ring 32. Thetrunnion Lipper half 48 is bolted to the trunnion lower half 47 tocomplete the pivotal mounting of the housing 12 (FIGURE l) on the twosupporting members 24, 25.

The leads 40, 49 are shown for representation of a source of electricalpower at a given, desired operating Voltage connectible, as described,to the electrodes 30, 31 for supplying the operating potential acrossthe gap between the electrodes. The operating voltage preferably isrelatively low, for example 40 to 70 volts, and the amperage accordinglyis high. The switch 50 is provided in the lead 40 for making andbreaking the electrical connection between the electrodes 30, 31 and theleads 40, 49 extending to the power source.

The modifications shown in FIGURES 2 and 3 include means, such as aradiation or optical pyrometer 51, responsive either to total radiationor to a particular portion of the spectrum of the energy emitted in thehot interior of the Crucible l0. In FIGURE 3, the temperature sensingmeans 51 is connected as by a linkage represented at 52 to the switchingmeans 50 and is responsive for opening the switch 50 when the furnaceinterior reaches a desired maximum temperature above 2800 F. and forclosing the switch 5t) when the furnace has lCooled to a desired lowerlimit above 2800 F. In both FIGURES 2 and 3, the pyrometer 51 receivesCrucible radiations through a vacuumtight sight hole 53 extending fromthe Crucible Cavity 11 to the exterior of the housing 12. A sight glassholder 54 (FIGURE 2) is attached in gas-tight manner in the outer end ofthe sight hole 53 and a sight glass is sealingly attached on its outerend. A gas, for example argon, is flowed into the sight glass holder 54near its outer end through an inlet tube 5S to keep furnace vapors sweptout of the sight hole 53, thereby preventing clouding of the sightglass.

The modification shown in FIGURE 2 employs a branched tube 22A leadingfrom the plenum chamber 16. One branch 56 leads, as in the othermodifications, to the argon supply through a valve 57. The other branch58 leads through a valve 59 to a supply of a gas other than argon (forexample, nitrogen) which may be used to dilute the argon supply in theCrucible chamber 1l. The pyrometer 51 is connected as at 60 to the argontiow regulating valve 57 to control the argon i'low as will bedescribed. Alternatively, the pyrometer S1 may be linked, as will becomeevident, with the valve 59 controlling the flow of the other gas. Eachof the valves 57, 59 thus constitutes a means for varying theconcentration of the argon in the Crucible Chamber 11.

To receive the Crucible contents 67, as described later, a mold isplaced on the housing upper side 17 as shown in FIGURE 3. Means areprovided for reducing atmospheric pressure, during any stage of furnaceoperation. in the Crucible chamber 11 as well as about the mold 61 andat the housing upper side 17, This means includes a compartment 62containing the mold 61 and formed by a top wall 63 which is associatedwith the housing top side 17 and extensions of the housing side wallsincluding the walls 18, 19. A tube 64 opening into this compartment 62leads to a vacuum pump or equivalent (not shown), and the compartment isclosed off, when desired, by a gas-tight door 65 (FIGURE 2).

In operation of the furnace for heating to a high temperature theenclosure 11 formed by the Crucible 10, an argon-enriched atmosphere isprovided and maintained in the Crucible. Utilizing the preferred meansfor accomplishing this step. the valve 57 (FIGURE l) is opened asrequired for directing a desired flow of argon through the tube 22 intothe plenum chamber 16, from whence it passes through the porous materialof the Crucible into the Crucible interior 1l. A pure atmosphere ofargon is not essential to plasma resistance operation, and the argoncontent can be varied widely as long as the minimum concentrationrequired for filling the furnace with a plasma resistor is maintained, apreferred minimum concentration being of the order of argon by weight.While, for reasons which will become apparent, it is preferred tointroduce the argon as described, other modes of introducing it areacceptable. As an example, sufficient argon may be tiowed into thefurnace through the sight hole purging tube (FIGURE 2), through anotherbore equivalent to the sight hole S3, or through a conduit Communicatingwith one or both the passageways 28, 29 (FIGURE 1) through which thepair of electrodes 30, 31 variably extend into the Crucible 10. Thesetting of the valve 57 is adjusted to obtain and maintain the desiredconcentration of argon during the remaining operation of the furnace.

A gas, in its normal state, is a very poor conductor of electricity andbecomes a conductor only when it contains enough free electrons and ionsto serve as carriers for a current. The zone including an ordinary arebetween electrodes Contains ionized materials which are of significantlyhigh carbon content where graphite electrodes are used and which arekept heated to ionizing temperature in part by resistance heating in theare but chiey by the heat emitted by the electrode tips. As theelectrodes are more widely separated, a gap is reached which is themaximum gap over which, ata given potential, an arc can be maintainedin, for example, air, carbon dioxide, nitrogen, etc. When this gap isexceeded. the arc breaks, for the ionization of the medium between theelectrodes becomes insuicient to maintain current flow between them. Themaximum gap is relatively small, and an arc cannot readily be maintainedover a gap exceeding about one-half inch at 40 volts or one inch atvolts, the gap being correspondingly smaller or greater as the voltageis decreased or increased. All the heating thus is localized to thesmall zone including the extreme tips of the electrodes and the narrowarc between them.

To provide more and other than ordinary arc heating, therefore, theargon-enriched atmosphere in the enclosure 11 provided by the Crucible10 is heated until some of it, generally throughout the enclosure, isthermally ionized. In this manner, the resistance between any two pointsspaced apart within the Crucible atmosphere is reduced, a reduction tothe order of 0.01 ohm per inch of spacing being sutiicient inrepresentative applications. Satisfactory ionization is obtained bybringing the cru- Cible atmosphere up to or above approximately 2800 F.,and this is done in `any convenient way resulting in the desiredCrucible interior temperature. The heating of this atmosphere iseffected, for example, by striking an arc between the electrodes 30, 31with a given. alternating or direct current potential across them andwith the electrodes spaced at least slightly less than the maximumspacing at which the arc can be maintained between them, at the givenpotential, in air or in the cold, argon-enriched atmosphere. The arc ismaintained until a temperature of the Crucible wall and interior isreached at which thermal ionization occurs in the argon of the Crucibleatmosphere outside the are. As measured by an optical pyrometer, thistemperature is very near 2S00 F.; a temperature of 2775o F., for exampleis not sufficient.

The argon-enriched atmosphere having been sutilciently heated, it isnecessary to provide a pair of electrodes in the enclosure. This willalready have been accomplished where, as described, the electrodes 30,51 are themselves utilized to provide preliminary heating by areoperation. Where the Crucible preliminary heating is brought about inanother manner, the electrodes 30, 31 nonetheless must be provided, inthis case in addition to preliminary heating means. Electrodes 30, 31 ofgraphite or of tungsten have yielded excellent results. Plasmaresistance operation is begun by increasing the spacing of` theelectrodes 30, 31 to provide a gap greater than the maximum gap overwhich an arc is propagable in air at the given potential. Ordinarily,the electrode spacing is several times, i.c., at least twice, thismaximum spacing, and actually a relatively wide spacing is desired toencourage current flow generally throughout the chamber 11 through thethermally ionized atmosphere tilling the same. For example, at anoperating potential of 4() volts, a 6-inch spacing between electrodes30, 31 is entirely feasible and is conducive of good results, A largergap may readily be employed, for example a gap of 5 inches or more foreach 25 volts of operating potential, and maximum gap size is ordinarilylimited, under lowvoltage, high-amperage operation, only by thedimensions of the Crucible chamber 11.

Since the ionized, gaseous resistance element or plasma substantiallyfills the entire chamber 11, separating the electrodes 30, 31 asdescribed and placing the given, selected voltage across them resultsnot in an ordinary arc but in a much more diffuse ow which occursthroughout the argon-enriched atmosphere in the enclosure 11. Since theflow is diffuse, it will be found that a potential (at least a largefraction of the operating voltage) is placed on a conductive probeplaced anywhere in the Crucible cavity 11 or even in the outflow ofplasma, like a tongue of flame, which ordinarily extends a shortdistance outside the Crucible opening 66. The entire atmosphere in theCrucible cavity 11 therefore is conductive. Before reaching atemperature at which plasma-resistance operation is possible, currentflow out of the electrodes 30, 31 is only at the electrode tips; currentdensity at the tips therefore is high, and the tips become very hot anddecompose with undesirable readiness. When all the atmosphere becomesconductive, current flow is out of` all the electrode surfaces (not.merely the tip surfaces.) within the furnace chamber 1l, and Currentdensity at the tips of the electrodes 30, 31 is radically lessened,although current flow remains fully as great as during arc operation atthe same voltage. Although the furnace temperature increases underplasma operation, electrode tip tern perature is markedly decreased andvaporization of the electrodes 30, 3l is radically reduced where notvirtually eliminated. Besides the advantages of greatly extendedelectrode life, carbon vapor contamination or adulteration of thematerial 67 heated in the furnace is obviated, and even under D.C.operation there is no carbon button built up on the anode and likely tofall into the melt. Most of the heat is generated by passage of theelectrical current through the plasma rather than originating at theelectrodes 30, 31; hence, this heat is generated throughout the Chamber11 rather than at and immediately between the electrode tips. As aconsequence, temperature is comparatively very uniform throughout thefurnace chamber 11 and there is little if any reliance on conduction,radiation. and convection for reducing temperature gradients within theCrucible 10. It is of interest that the current flow through the plasmaresistor differs in kind from that through an arc. As shown in FIGURE 5,the How through electrodes separated by a gap spanned by an arc, an A.C.flow being shown by way of example, results in or is accompanied bysharp voltage fluctuations (voltage across the electrodes being shown bythe line 70) which greatly vary the waveform of the power supplyvoltage, in this case a sine wave. FIGURE 6 shows the voltage acrosselectrodes to which an AC. current is fed during plasma resistanceoperation. Without entering into discussion of the causes of the rapidand violent variations of voltage during arc operation, it will be notedthat the voltage during plasma resistance operation follows the puresine wave of the power source. The plasma behaves, therefore, purely asa resistor of constant value. The are flow is of another kind andnature, as evidenced by the voltage fluctuations.

The plasma resistor, resulting from ionization within an atmosphereenriched with one of the noble, inert gases, offers none of the problemspresented by other resistors. The plasma is entirely compatible with allmaterials which may be employed in constructing the furnace or heater'in the same; it does not chemically attack other materials, nor is ititself oxidized or otherwise affected by air. In fluid state, it is notadversely affected by further increase in temperature above 2800 F. andhence is not subject to a melting or vaporization such as that which. infurnaces employing a solid or liquid resistor, limits the upper range ofoperating temperature. There are no resistor maintenance problems, itbeing ncessary only to ensure an adequate concentration of theplasmaproducing gas in the Crucible. Operating pressures are in no waycritical. as in an electron beam furnace: the plasma resistor has beenoperated efficiently from pressures above atmospheric down to pressuresas low as 0.03 mm. of mercury.

Furthermore, it is important to note that the argon serves not only toprovide the plasma for heating, it also protects the furnace interiorand its contents from reaction with atmospheric gases and furthermoreserves the important and valuable function, entering the chamber as itdoes through the porous Crucible material at the bottom of the Cruciblechamber, building up through the 8 heated material 67 and scrubbing thesame of impurities when it is melted as in FIGURE 3.

Whereas an arc furnace is limited to the maximum temperature attainableat the heated material by heat brought to the latter, from the are zone,by conduction, convection, and radiation, no such limitation exists whenheating with the plasma resistor, for heating occurs throughout thefurnace cavity. The furnace operating temperature therefore is limitedonly by the ability of the Crucible material to withstand melting.lvfeanwhile, it will be seen that temperature control when using theplasma resistance element is `more effective and more easily attainedthan in arc and electron beam furnaces and is comparable withtemperature control in solid and liquid resistor furnaces.

According to a preferred feature of the method of operating an electricfurnace whereby the furnace interior 11 is maintained within a desiredtemperature range whose upper and lower limits lie abov 2800 F., thecurrent flow through the electrodes 3E), 31 and plasma is interruptedwhen the temperature of the furnace interior reaches the desired upperlimit. This is readily accomplishcd by opening the switch 50.

The furnace is then allowed to cool for a period of time sufficient forits temperature to be reduced to the desired lower limit. At this time.while maintaining the above-described electrode spacing for plasmaresistor operation, the given operating voltage is again applied acrossthe electrodes 30, 31 to resume the Current flow. Closure of the switch50 is sufficient to effect this flow, for while the furnace interior 11remains above 280W F., the argon-enriched atmosphere within the furnaceremains conductive and current flow commences immediately upon placingthe operating voltage across the electrodes. There is no need. as inordinary arc furnace operation, for touching the electrodes 30, 31together or for added equipment for providing a high-voltage startingspark between spaced electrodes. The sole requisite for operation, asabove described, for controlling temperature within the electric furnaceis that the current ow be resumed before the argon-enriched atmospherein the furnace interior has cooled below 2800" F., for upon passingbelow this temperature the Crucible atmosphere is no longer sufficientlyionized to re-start the flow without resort to touching the electrodes30, 31 together or reducing their spacing to an ordinary arc gap andcmploying a high, special starting voltage.

Temperature within the electric furnace also is effectively controlledby increasing the argon content of the Crucible atmosphere to raise theCrucible temperature and decreasing the argon content to lower thetemperature. While the Crucible 10 is operating, for example, with theCrucible opening 66 exposed to the atmosphere, relatively cool and henceheavy air outside the Crucible tends to flow down through the opening 66and thus to dilute the argonenriched atmosphere in the Crucible. Agiven, constant flow of argon into the Crucible through the tube 22 or22A therefore will tend to result in an equilibrium being reached atwhich the argon content in the Crucible l() is relatively constant.Adjusting the valve 57 to provide a greater argon tlow further enrichesthe Crucible atmosphere and concurrently raises its conductivity. Theresulting increase in amperage of the current passing through thecrucible atmosphere, voltage being held constant, increases the rate ofevolution of heat. Dilution of the Crucible atmosphere with air or othergases increases its resistance and diminishes the current flow, therebylowering the furnace temperature. T he argon content must not be loweredso greatly as to lose the plasma resistance mode of operation, and ithas been found that this operation is still well retained when the argoncontent has dropped to 15%. By the same token, the temperature reductioncannot proceed below 2800" F., for the plasma resistance operation willbe lost below this temperature.

Automatic temperature control is obtained by operation of themodification shown in FIGURE 3. The temperature sensing means, forexample the optical pyrometcr l, senses the temperature within theCrucible 10 through the sight hole 53 and responds to occurrence of themaximum desired temperature by opening the switch 5t) by actuating thelinkage 52. In response to the furnace tem perature having fallen to thedesired lower limit, the temperature sensing means actuates the linkageto close the switch and thus to re-start the current ow.

In the modification shown in FIGURE 2, the temperature sensing meansresponds to temperatures Within the crucible chamber to open and closethe argon How control valve 57 through the linkage 60, the valve beingmore Widely opened to increase argon flow when the `furnace temperaturefalls below a desired value and closed to decrease argon flow when thefurnace temperature becomes excessive. Alternatively', the linkage 60can be connected to the valve 59 in the branch 58, whereupon the argonflow is set manually to a constant value and the argon concentration inthe crucible varied by action of the sensing means 51 on the secondvalve 59. The second valve 59 is opened to admit a gas other than argon(for example, nitrogen) through the branch 58 and thus to dilute theargon ow into the crucible chamber l1. The dilution results in alessened current flow and lowered temperature. The dilution must not beexcessive, for too much nitro-gen, air, carbon dioxide, etc. will soreduce the argon concentration and increase the resistance of theCrucible atmosphere as to result in abrupt loss of plasma resistanceoperation. Closing the valve 59 permits the argon concentration to riscand thus raises the temperature in the crucible 10.

Whenever operation of the furnace under a partial vacuum is desired, themold compartment 62 is rendered airtight by closing the door 65 (FIGURE2). The caps 35, 36 are sealingly mounted on the electrode holders atthe fittings 26, 27 to prevent the inflow of air around the electrodes30, 31 and the compartment 62 and crucible chamber 11 are evacuated, tothe extent desired. by connecting the tube 64 to a vacuum pump orequivalent. According to one frequently employed sequence of operations,the metal 67 (FIGURE l) or other material to be heated is introducedinto the furnace and melted at atmospheric pressure with, of course,argon enrichment. The mold 61 (FIGURE 3) then is placed on the housingupper side 17 and in register with the Crucible pouring hole 66. Asupport plate 68 is placed on top of the mold 61, and the mold isclamped in place by fastening means such as a pair of air-drivenactuators 69 whose piston rods are extensible, upon air being suppliedto the actuators, into engagement with the support plate. The door 65(FIGURE 2) is p-ut in place to render the mold compartment 62 (FIGURE 3)airtight, and the compartment 62 and crucible chamber 11 are evacuatedthrough tube 64. After further heating of the melt 67, where desired,the housing 12 is rotated 18() degrees on the supporting members 24, 25,thereby pouring the liquid metal 67 (FIGURE 3) into the cavity of themold 61. After the metal has solidified, the furnace is rotated back toits original position. After introducing argon to return the Cruciblechamber 11 to atmospheric pressure, the gas-tight door 65 is opened andthe mold is removed from the compartment.

While only certain particular embodiments and modications of theinvention have been described herein and shown in the accompanyingdrawing, it will be evident that further modifications are possiblewithout departing from the scope of the invention.

We claim:

1. The method of heating an enclosure comprising:

providing and maintaining in the enclosure at atmosphere of at leastargon content;

heating substantially all the atmosphere in the enclosure to atemperature at which some of the argon of said atmosphere is thermallyionized throughout the enclosure;

providing a pair of electrodes in the enclosure and spacing them apartto provide between them a gap greater than the maximum gap over which anarc is propagable between the electrodes in air at a given potential;

and applying said given potential across the gap to obtain a flow ofelectrical current through the atmosphcre in the enclosure.

2. The method of heating an enclosure comprising:

providing and maintaining an argon-enriched atmosphere in the enclosure;

heating the atmosphere in the enclosure to a temperature at which theelectrical resistance of the atmosphere is lowered to the order of 0.01ohm per in.h between spaced points substantially anywhere in theatmosphere by ionization of argon of the atmosphere;

providing a pair of electrodes in the enclosure spaced to provide a gapbetween them greater than the maximum gap capable of supporting an arcbetween the electrodes in air at a given potential:

and applying said given potential across the electrode gap to obtain adiffuse llow of electrical current through the argon-enriched atmospherein the enclosure.

3. The method of operating an electric furnace having a Crucible and apair of electrodes variahly extensible into the Crucible, said methodcomprising:

providing and maintaining in the Crucible an atmosphere of at least 15%argon content; placing a given potential across the electrodes andproviding an arc between them by spacing them at a given spacing lessthan the maximum spacing :it which, at the given potential. an arc canbe maintained between them before the atmosphere in the Crucible isheated;

heating the Crucible by maintaining said arc until the crucible interiorreaches a temperature at which ionization occurs in the argon of saidatmosphere well outside the arc;

and increasing the spacing between the electrodes beyond said maximumspacing. 4. The method of operating an clcctric furnace haring adielectric crucihie, said method comprising:

providing a pair of electrodes made of a material selected from thegroup including carbon and tungsten and variably extensible into theCrucible;

maintaining in the Crucible an atmosphcrc of at least 15% argon content;

providing an arc `between the electrodes b v placing a igiven,alternating potential across the electrodes und spacing them at a givenspacing no greater than the maximum spacing at which an arc can bemaint-.lined between them. at the given potential, before the atmospherein the crucible is heated;

heating thc Crucible and its contents by maintaining said arc until atemperature is attained at which iunization occurs well outside the arcin the argon of the atmosphere in the cruciblc;

and increasing the spacing between the electrodes to n `spacing whichexceeds twice said maximum spacing.

5. The method of operating an electric furnace coinprising:

providing and maintaining in the furnace an argonenriched atmosphere;heating the furnace interior and substantially all the argon-enrichedatmosphere to a temperature in excess of 280W F.;

providing a pair `of electrodes in the furnace spaced by a gap of morethan 5 inches;

and passing a flow of electrical current through thc argon-enrichedatmosphere by applying a power source with a potential of less than 200volts across the gap between the electrodes.

6. The method of maintaining the interior of an electric furnace withina desired temperature range having an upper and a lower limit both above2800 F., said method comprising:

maintaining an argon-enriched atmosphere in the furnace;

providing in the furnace a pair of electrodes;

heating the furnace interior in excess f 2800" F.;

passing a flow of electrical current through the argoneurichedatmosphere by applying a given potential across the electrodes with theelectrodes spaced a given spacing which is a plurality of times thespacing across which an arc can be maintained at the given voltage inair;

interrupting the current flow through the argon-enriched atmosphere whenthe furnace interior reaches the upper limit ofthe temperature range;

and leaving the electrodes spaced at said given spacing and applyingsaid given voltage across them to resume the flow of electrical currentthrough the argon-enriched atmosphere before the furnace interior hascooled to the lower limit of the temperature range.

7. The method of controlling the temperature within an electric furnacehaving variably spaceable electrodes comprising:

heating the furnace interior above 2800 F. by striking an arc betweenthe electrodes;

maintaining an argon-enriched atmosphere in the furnace;

spacing the electrodes to provide between them a gap which is aplurality of times the maximum gap at which an arc is sustainablebetween the electrodes in air at a given potential;

passing a flow of electrical current through the argoneurichedatmosphere by imposing said given potential across the electrodes;

and increasing the argon content in the furnace atmosphere to raise thetemperature Within the furnace and decreasing the argon content in thefurnace atmospherc to lower the temperature within the furnace.

8. The method of operating an electric furnace having a dielectricCrucible and a pair of electrodes variably cxtcnsible into the Crucible.said method comprising:

maintaining an argon-enriched atmosphere in the crucible; heating theCrucible interior, including the argon-enriched atmosphere, above 2800F.;

and passing an electrical current through the argoncnriched atmosphereat a given potential with the electrodes separated by a gap of at leastone inch for each volts of said potential.

9. The method set forth in claim 8 and further comprising:

cutting off the electrical current How through the argonenrichedatmosphere when the furnace temperature reaches a desired upper limitabove 2800 F.; and

maintaining said electrode gap and resuming the current ow through theargon-enriched atmosphere by applying said given potential across thegap when the furnace temperature has fallen to a desired lower limitabove 2800a F.

l0. The method of heating an enclosure comprising:

providing and maintaining in the enclosure an atmosphere enriched with anoble gas;

heating the atmosphere in the enclosure to a temperaturc at whichsubstantial ionization occurs in the noble gos throughout the enclosure;

positioning a pair of electrodes in the enclosure and spacing them apartto provide between them a gap greater than the maximum gap capable ofsupporting an arc between the electrodes in air at a given potential;

and applying said potential across thc electrode gap to obtain a diuseflow of electrical current through the ionized, noble gas enrichedatmosphere in thc cnclosure.

Il. The method of heating on enclosure containing an atmosphere, saidmethod comprising:

enriching the atmosphere contained within thc cnclosurc with o noblegus;

heating the enclosure interior until substantial ionization has occurredsubstantially throughout the enclosure interior in the noble gasenriching thc cnclosure atmosphere;

positioning in the enclosure n pair of electrodes;

and passing a diuse flow of electrical current through the ionized,enriched atmosphere by applying n given potential across the electrodeswith the electrodes spaced a given spacing which is n plurality oftintes thc spacing across which on arc can bc maintained at the givenvoltage in air.

References Cited by the Examiner The following references, cited by theExaminer, are

of record in the patented le of this patent or the original patent.

UNITED STATES PATENTS 1,310,079 7/1919 Hechenbleikner 13-9 1,499,9227/1924 Hadaway 13-9 2,147,070 2/1939 Weinheimer 13-34 2,285,837 6/1942Ridgway 13-34 2,726,278 12/1955 Southern 13-9 2,782,245 2/1957 Preston13-9 3,004,137 10/1961 Karlovitz 219-75 FOREIGN PATENTS 152,176 10/1920Great Britain.

RICHARD M. WOOD, Primary Examiner.

JOSEPH V. TRUH E, Examiner.

